An electrostatic image developing toner is used in image formation of visualizing electrostatic images in printers, copiers, facsimiles, etc. An example of image formation through electrophotography is referred to, in which an electrostatic latent image is first formed on a photoreceptor drum, and then this is developed with a toner, transferred onto transfer paper or the like, and fixed by heat or the like for image formation thereon.
As an electrostatic image developing toner, in general, used is one having such a form that is prepared by adhering an external additive of, for example, solid fine particles of silica or the like to the surfaces of toner particles obtained according to a so-called melt-kneading and grinding method that includes dry-mixing a binder resin and a colorant and optionally an electrification control agent, a release agent, a magnetic substance and the like, then melt-kneading them in an extruder or the like, and thereafter grinding and classifying the resultant substance, for the purpose of giving various properties such as flowability and the like to the toner particles.
Recently, in image formation in copiers, printers and the like, it has become desired to provide high-definition high-quality images, and there have been proposed various polymerization methods such as a suspension polymerization method, an emulsion aggregation method, a dissolution suspension method, etc., which easily controls the particle size of toner particles and the particle size distribution.
Further, with the recent popularization of copiers, printers and the like a toner has become desired that is excellent especially in high-speed printing performance and low-energy fixation capability in addition to the requirement to image quality, and improvement of low-temperature fixation performance of toner has been tried. For attaining low-temperature fixation, much used are a method of lowering the glass transition point of a binder resin, and a method of additionally using a crystalline resin, but low-temperature fixation performance and blocking resistance or hot offset resistance are contradictory to each other, in general and it is desired to satisfy both the two.
Regarding these problems, there has been tried a method of maintaining blocking resistance of toner while maintaining low-temperature fixation performance thereof, by employing a core-shell structure in which a shell layer having a high glass transition temperature (Tg) and excellent in heat resistance is formed around the surface of a core formed of a resin excellent in low-temperature fixation performance and having a low melt viscosity.
It is known that, in forming a core-shell structure, when a method of heating the structure at a high temperature after shell particles have been adhered thereto is employed, the shell particles are buried simultaneously with advance of fusion of core particles and shell particles and, as a result, there may form a non-coated part to cause insufficient blocking resistance. In addition, it is also known that, when the shell component is too large, the low-temperature fixation performance of toner would be thereby disturbed, but on the contrary, when the shell component is too small, a non-coated part may form and the core component may be thereby exposed to the toner surface, and therefore, the expected blocking resistance performance could not be obtained.
PTL 1 has tried satisfying both low-temperature fixation performance and cleaning performance, in which the core mainly contains a crystalline resin, the shell accounts for from 15% by mass to 120% by mass, more preferably from 25 to 100% by mass, even more preferably from 35 to 80% by mass relative to the core, and the shell has semispherical projections having a height difference of 0.3 μm or more. PTL 2 has tried satisfying both fixation capability and heat resistance, in which an interlayer containing inorganic fine particles or organic fine particles is formed on the surface of the inner core particle of toner and an outer shell layer is formed around the surface of the layer. PTL 3 has tried satisfying both low-temperature fixation performance and heat resistance storability by providing core-shell particles in which the core particle is surrounded by a shall layer composed of a resin particle layer A for securing heat resistance storability and a resin particle layer B existing around the layer A for securing emulsion stability. PTL 4 has tried evenly adhering an electrification control agent or an electrification control resin to the surface of a toner by making the surfaces of mother particles hold a positively-charging compound therearound and further firmly fixing a negative electrification control resin fine particles around the surfaces thereof.